A species of Argentine ant introduced into Europe about 80 years ago has developed the largest supercolony ever recorded.

It stretches 6,000 kilometres - from northern Italy, through the south of France to the Atlantic coast of Spain - with billions of related ants occupying millions of nests.

While ants from rival nests normally fight each other to the death, ants from the supercolony have the ability to recognise each other and co-operate - even if they come from nests at opposite ends of the colony's range.

The Argentine species (Linepithema humile) probably came into Europe on imported plants, pushing back the 20 or so indigenous species of European ant.

Internal strife

Scientists are not entirely sure why the supercolony has emerged.

Ant facts

More than 4,500 recorded species

Live in highly organised colonies

Dominated by infertile female worker ants

They maintain the colony by searching for food and nursing the young

Only the winged female (queen) can mate

Males do little other than mate with the queen and die soon after

They think the initial success of the alien invaders would have led to high nest densities, which in turn would have favoured co-operative behaviour over aggression.

And evolution would then have reinforced this superiority because nests devoid of internal strife would have had time and resources to fight off their enemies.

Success would have bred success.

"It is interesting to see that introduction in a new habitat can change social organisation," said Professor Laurent Keller, of the University of Lausanne, Switzerland, and one of the scientists to have identified the supercolony.

"In this case, this leads to the greatest co-operative unit ever discovered."

Smell the same

But Professor Keller and colleagues say the supercolony may be doomed. Sooner or later, rivalries will emerge as genetically distinct groups of ants turn against each other.

The supercolony itself also has a rival - a second, smaller supergroup of Argentine ants holds sway in the Catalan region of Spain. These creatures are more than happy to make war.

The ants would have come into Europe on imported plants

The research, by Swiss, French and Danish scientists, is published in The Proceedings of the National Academy of Sciences.

Professor Jurgen Heinze, an expert on ants at Erlangen University, Germany, is impressed with the work. He told the BBC there must be some important genetic similarities running through the colony.

"An ant has to decide when encountering another individual whether this individual belongs to its colony or not. The recognition cues are odours on the surface of the ant and these odours are in part genetically based.

"Genetic variation leads to variation in the recognition cues and if there is a loss of genetic variation, the ants all smell alike and they can no longer distinguish between alien ants and nest mates."